To establish communication among telephone instruments, a method of connecting the desired stations is needed. The original direct connections quickly gave way to the switching systems. The basic concept of telephone switching systems is to centralize the routing process by utilizing shared resources. The service is then delivered to the destined subscribers through individualized direct cabling. The main advantage for this is to provide the "addressability" to each subscriber so that they can assume their individual identity. This paved the way to the modern direct dialing telephone network. One drawback of this architecture is that a connection between two physically adjacent stations have to be done by wiring route that traverses from one station to the central switching system, then travels back practically the exact same route again to the second station. Obviously, it would be advantageous to use only one single run of wiring to connect both stations, especially, if these stations are far away from the switching equipment with the traffic to and from these stations from other subscribers being generally rather light compared to that between them. The party-line service was a good example of a classical engineering achievement in reducing telephone wiring for rural areas. Although there were certain operational limitations, it demonstrated the economical benefits of providing the basic telephone service to remote subscribers. Another example is at private residences, where multiple station sets may be installed in different rooms throughout the premises and connected by "parallel" wiring. This provides the convenience of being able to use the telephone service without being restricted to one particular room. However, there are certain disadvantages:
First, although a call can be originated from any location where an instrument is available, an incoming call would also indiscriminately alert all instruments at the same time. A wrong number call in the middle of the night often makes the person who gets waken up wonder whether having a telephone set in the bedroom was a right idea in the first place. Also, with more than one person on the same premises that normally expects receiving telephone calls, it is distracting to everyone when the phone rings. Who should answer it is the first reaction. More than often, the call is not for the person who eventually picks up the receiver. On the other hand, because of the telephone company's equipment performance criteria, a maximum of five phones with active bell connections is allowed on the same line. This restricts some of the station instruments from having their ringers connected.
In addition, there is no way to use a telephone set in one room to alert and then to converse with a person in another room, even though both rooms are equipped with telephone sets.
Furthermore, there has been increasing number of special purpose station instruments such as Telephone Answering Device (TAD), facsimile (FAX), computer modem, security alarm, etc, in recent years. These instruments offer diversified telecommunication capabilities. However, for uniformity in interface, each of these devices is designed to be used directly on a telephone line. Without proper installation arrangement, each of these instruments would occupy a subscriber line even though they are in standby mode most of the time. With subscribers becoming more sophisticated and utilizing various telecommunication equipment, many have to subscribe to more than just one single line service from the telephone company. Since each line requires a telephone number to be identified by the switching system in providing the service, this has contributed, at least partially, to the rapid depletion of the available telephone numbers in the national numbering plan.
In the business environment, a Private Branch exchange (PBX) switching machine providing the local concentration function can relieve some of these problems. A PBX can have more station instruments installed and be in use at a given time than the number of lines available from the telephone company. My Modular Automatic Telephone Switching System (MATSS) (U.S. Pat. No. 5,022,069) further offered an economical solution to the above applications for very small telephone installations. At its simplest form, a stand-alone module called Master Switching Unit (MSU) could be set up to supervise a single line telephone service with the equivalent function of a PBX. Compared to the conventional PBX, the MATSS concept even has an integral automated attendant function which has become increasingly important for small businesses who need to project the same professional image as the large companies, but can not afford the expense of an operator, a receptionist or a full sized automated attendant equipment. Thus, the MATSS can be classified as a PABX (Private Automatic Branch exchange).
The single line MSU approach has been proven to be very effective for a small business. However, one physical limitation during installation remains an impedance to the general acceptance of it: Since the wiring for all stations on a single line service is traditionally "parallel", it is not possible for the MSU to treat each station with individual identity, unless they are re-connected to the MSU via individualized direct cables. Although it can be done quite straightforwardly, this subject is difficult for residential subscribers to comprehend and to actually put in new cables for the direct wiring between the MSU and the individual telephone outlets.